Hello,

I intend to control a valve for mixing water at two different temperatures (hot and cold) for a shower. However, since I'm new to programming and Step/Siemens, its being very difficult to make the PID control to it (I have another topic with doubts with that), therefore it appeared some probably very basic doubts, such as:

- From what I read in the manual, forum and examples, PID should be called in a cycle interrupt. This sould always be done? Can it be placed in the main cycle? Its just because I need to have several things being done at the same time as the temperature control, that is, while I'm controlling the temperature with the PID I needed for example to open several valves and operate several fans and motors (relay set by the HMI). 

Since I'm using the PID that is not possible because the cycle is interrupted, is it? Or i figured wrongly?

- By the way, the PID cycle interrupt is always being called since I turn the PLC or I can put a condition on it? for example to have water entering or pushing a button?

Thanks ind advance for the help

bruno_cardoso

1- From what I read in the manual, forum and examples, PID should be called in a cycle interrupt. This sould always be done? Can it be placed in the main cycle? Its just because I need to have several things being done at the same time as the temperature control, that is, while I'm controlling the temperature with the PID I needed for example to open several valves and operate several fans and motors (relay set by the HMI). 

Since I'm using the PID that is not possible because the cycle is interrupted, is it? Or i figured wrongly?

2- By the way, the PID cycle interrupt is always being called since I turn the PLC or I can put a condition on it? for example to have water entering or pushing a button?

 Hello ,

For your doubts regarding to PID controllers:

1. PID controllers are compulsory to be called in a cyclic interrupt with a known cycle.This is necessary for optimized/accurate operation of PID controller due to math inside it.Cycle is like time difference in Calculus and it is necessary for numeric calculations.All cyclic interrupts has priority that determines the level of importance to user, when it is high it can not be interrupted by less important tasks.
   
   
2. I don't recommend to disable PID controller based on event, just put in manual mode and a safe  state in output.
   

I hope this helps,

hdhosseini

hdhosseini

 

bruno_cardoso

1- From what I read in the manual, forum and examples, PID should be called in a cycle interrupt. This sould always be done? Can it be placed in the main cycle? Its just because I need to have several things being done at the same time as the temperature control, that is, while I'm controlling the temperature with the PID I needed for example to open several valves and operate several fans and motors (relay set by the HMI). 

Since I'm using the PID that is not possible because the cycle is interrupted, is it? Or i figured wrongly?

2- By the way, the PID cycle interrupt is always being called since I turn the PLC or I can put a condition on it? for example to have water entering or pushing a button?

 Hello ,

For your doubts regarding to PID controllers:

1. PID controllers are compulsory to be called in a cyclic interrupt with a known cycle.This is necessary for optimized/accurate operation of PID controller due to math inside it.Cycle is like time difference in Calculus and it is necessary for numeric calculations.All cyclic interrupts has priority that determines the level of importance to user, when it is high it can not be interrupted by less important tasks.
   
   
2. I don't recommend to disable PID controller based on event, just put in manual mode and a safe  state in output.
   

I hope this helps,

hdhosseini

Hello,

First of all thank you for the information given. It would be possible to help me on this matter? I still have several doubts:

- From what I understand the PID should be called in a cycle interrupt to function properly, but how do I call it in the Main Cycle? Or he is always working?

- When the PID (cycle interrupt) is working this means that I can not use more functions of my code in the Main Cycle? Like press buttons on the HMI to trigger other things such as pumps? Its just because in my program I do more than control the temperature of the water and do not know if when I'll be doing this if the PLC/program lets me do more things at once or do just that.

- I must no disable the PID? But if I just need the PID to control the temperature when I turned on the water, therefore I only need it when that happens. During the rest of the time i can not turn off the PID (and the Siemens valve) too? or it should be always working?

Sorry if my questions are silly but I am very new at PLC programming and I have almost no knowledge of it but I'm trying to learn from those who know.

By the way, when I'm doing the PID comissioning the system must be turned on, this is, with the running water and everything, right?

Once again thank you

Maybe it is not needed. Please step one step back with me.
Allow me to cover two basic principles briefly in a simple way..

[1]-------------------------------------------------------------
PID controller is detailed in [LINK].


Among other things the following pseudocode is given:

previous_error = 0
integral = 0
start:
  error = setpoint - measured_value
  integral = integral + error*dt
  derivative = (error - previous_error)/dt
  output = Kp*error + Ki*integral + Kd*derivative
  previous_error = error
  wait(dt)
  goto start 

OB100 //First scan
previous_error := 0;
integral := 0;

OB1 //Main cycle
{machine / system control}

OB32 //Time cycle interrupt; repeated every 100ms
// example code follows..
// this should be replaced with actual PID
  dt := 0.1; //cycle repeat
  error := setpoint - measured_value;
  integral := integral + error*dt;
  derivative := (error - previous_error)/dt;
  output := Kp*error + Ki*integral + Kd*derivative;
  previous_error := error;

 [2]-------------------------------------------------------------

Please do not fear using the interrupt functioning. It will do what you need.

With that I leave you in the expert's hands to cover the technical stuff. (smile)

w

bruno_cardoso

I must no disable the PID? But if I just need the PID to control the temperature when I turned on the water, therefore I only need it when that happens. During the rest of the time i can not turn off the PID (and the Siemens valve) too? or it should be always working?

 You don't need to disable PID regulator, you only need to keep it in manual mode when you don't want it to automatically calculate the result.Let the PID keeps running and design your controller in such a way that manual-->auto-->manual mode changes are bumpless !

hdhosseini

 

bruno_cardoso

I must no disable the PID? But if I just need the PID to control the temperature when I turned on the water, therefore I only need it when that happens. During the rest of the time i can not turn off the PID (and the Siemens valve) too? or it should be always working?

 You don't need to disable PID regulator, you only need to keep it in manual mode when you don't want it to automatically calculate the result.Let the PID keeps running and design your controller in such a way that manual-->auto-->manual mode changes are bumpless !

 

Hi hdhosseini,

This mean that when I want to start the shower I should use a botton (variale on the HMI) to activate the Mode Activate (PID) and set the Mode (PID) to automatic. And when I finish the shower I should activate the Manual Enable (PID) and set the Manual Value (PID) to a more or less middle value of the position signal of the valve. Is it, right?

With the manual value activated the value of the thermocouple (process value) will not be compared with the setpoint and will not move the valve, right? Just confirming, sorry.

Thanks for your help 

William_B

Maybe it is not needed. Please step one step back with me.
Allow me to cover two basic principles briefly in a simple way..

[1]-------------------------------------------------------------
PID controller is detailed in [LINK].


Among other things the following pseudocode is given:

previous_error = 0
integral = 0
start:
  error = setpoint - measured_value
  integral = integral + error*dt
  derivative = (error - previous_error)/dt
  output = Kp*error + Ki*integral + Kd*derivative
  previous_error = error
  wait(dt)
  goto start 

PID controllers in a PLC use a sampling method and (discrete) algorithm. At regular intervals a measurement is made. By mathematical formula this sample measurement is compared to what is asked (setpoint) and what has happened before. The resulting output try to control the process with a "prediction" of what is required at that moment to aim to the requested setpoint.

The digital version of the PID keeps in mind what is the time intervals between sampling/calculations. That time is the "dt" in the pseudocode above. It is preferred to have "dt" a fixed period. The scan cycle of OB1 cannot give us a good constant time period, where an interrupt can. In the PLC's case you will have..

OB100 //First scan
previous_error := 0;
integral := 0;

OB1 //Main cycle
{machine / system control}

OB32 //Time cycle interrupt; repeated every 100ms
// example code follows..
// this should be replaced with actual PID
  dt := 0.1; //cycle repeat
  error := setpoint - measured_value;
  integral := integral + error*dt;
  derivative := (error - previous_error)/dt;
  output := Kp*error + Ki*integral + Kd*derivative;
  previous_error := error;

 [2]-------------------------------------------------------------

An interrupt routine does not stop the PLC from functioning.
You could imagine the following happening:
* OB1 related tasks are busy with something
* interrupt to OB1: "remember what you were doing"
* interrupt to OB1: "halt your process for a little while"
* interrupt does its job
* interrupt to OB1: "remember what you were busy with?.. continue with that"
* OB1 related tasks continue as if nothing happened.

You may have several interrupts for your main cycle and the PLC will still execute all the code. Trouble comes when the PLC spends too much time on interrupts or when interrupts overlap.

Please do not fear using the interrupt functioning. It will do what you need.

With that I leave you in the expert's hands to cover the technical stuff. (smile)

w

Hi William_B,

Firstly thank you for the help and the explanation 

Now I realize why the cycle interrupt, this is, the PID must always be active for it to work properly when necessary to control the temperature.

But for example can I use a boolean variable as a condition to active the PID (in the cycle interrupt)? Its just because my system will always be active (24h per day) and I only will use the shower/PID for 8 hours. Can i turn it off during the rest of the time to save the valve and increase its service life?

I think I must only use one interrupt (or at least hope) in my program and therefore should not occur overlap. You said that the cycle would halt the OB1 a while, whis will not stop the remaining functions that are running and controlling other processes, will it? Ouputs (relays and others) will continue to operate or will they stop for that period?

Thanks again for everything

bruno_cardoso

...
Now I realize why the cycle interrupt, this is, the PID must always be active for it to work properly when necessary to control the temperature.
...

You already have the answer, my friend (smile):

hdhosseini

 Let the PID keeps running and design your controller in such a way that manual-->auto-->manual mode changes are bumpless !

Please allow me to explain..
When you have manual mode active the automatic PID loop is not longer in control. You (the PLC code) decide what the output value should be.

So,, when the PID is set to manual mode you can make the output zero / off. The output will then do nothing. Interrupting to the interrupt should not really be an issue.

Please see the topic "Description of PID_Compact" under your TIA Portal help files. There is an image that may be of value to you. Especially the "manual" portion.

Does that help?
w